Corrosion can occur on the outside of a pipe (due to corrosive soil) or on the inside of a pipe (due to corrosive water.) We will be most concerned with internal corrosion, although external corrosion is a similar process and can also cause problems in the distribution system.
Either outside or inside a pipe, corrosion can have one of
Each cause somehow sets up an anode and a cathode so that corrosion can
occur. The creation of the
corrosion cell can be through electrolysis, oxygen concentration cells,
or through galvanic action.
In electrolysis, a
electric current enters a metal pipe and causes flow of electrons
through the pipe and to the ground. The pipe, fueled by the
electric current, becomes the anode while the soil becomes the
cathode. The outside of the pipe corrodes, with the metal
from the pipe plating out in the surrounding soil.
Electrolysis can occur when D.C. electric currents are grounded onto pipes. Nearby electric transit systems can also cause electrolysis.
Oxygen Concentration Cell
More commonly, the water and its constituents may set up a corrosion cell within the pipe. These corrosion cells, known as oxygen concentration cells, result from varying oxygen concentration in the water. The portion of the pipe touching water with a low oxygen concentration becomes the anode while the part of the pipe in contact with a high oxygen concentration becomes the cathode.
Oxygen concentration cells are probably the primary cause of corrosion in the distribution system. They may occur at dead ends in the distribution system where water is stagnant and loses its dissolved oxygen. Alternatively, oxygen concentration cells may begin in annular spaces, which are ring-shaped spaces between two pipes or between a pipe and a pipe lining. In every case, oxygen becomes depleted in these regions since they are cut off from the normal flow of water, so a difference in oxygen concentration is set up between the dead end or annular space and the main flow of water.
Oxygen concentration cells can also be caused by bits of dirt
bacteria. Both of these can become attached to the pipe walls,
shielding the metal from dissolved oxygen in the water and setting up
Metals themselves can also set up corrosion cells. When a pipe consists of only one type of metal, impurities in the pipe wall can develop into anodes and cathodes. Alternatively, when two dissimilar metals come into contact, galvanic corrosion will occur. Galvanic corrosion is often set up in the distribution system in meter installations and at service connections and couplings.
The galvanic series,
below, arranges metals according to their tendency to corrode.
series can be used to determine whether galvanic corrosion is likely to
occur and how strong the corrosion reaction will be.